1. Field of the Invention
This invention relates to a stamp detecting device, stamp detecting method, letter processing apparatus and letter processing method for detecting a stamp affixed to a postal letter, for example.
2. Description of the Related Art
Further, this invention relates to a letter processing apparatus and letter processing method for sorting a postal letter by use of the above stamp detecting device.
In a stamp detecting device, as various threshold values (for example, a threshold value used for distinguishing the stamp area and the ground area of the letter) used for detecting a stamp affixed to a postal letter, predetermined values or values derived by a statistical calculation based on an image of the letter are used when the stamp is detected based on a density projection value.
Further, when the type of stamp affixed to the postal letter is identified, the identification method which is not influenced by the affixed orientation of the stamp is used. Also, when the type of stamp affixed to the postal letter is identified, the identification method is effected without using knowledge of the face value of the stamp.
However, in the above conventional methods, it is difficult to adequately set the threshold value used for detection of the stamp and if the threshold value is not adequately set, it fails to correctly detect the stamp in some cases.
Further, the identification method which does not depend on the affixed orientation of the stamp has a problem that the identification performance is lower than that of the identification method which depends on the affixed orientation of the stamp. In addition, since it does not utilize the knowledge of the face value of the stamp, there occurs a problem that a lot of identification errors are made.